Variable gain logic for a GPS based automatic steering system

ABSTRACT

A method is provided for controlling steering oscillations in a work vehicle having a closed loop GPS based automatic steering system, wherein the closed loop steering system has a default gain and a user defined gain. The method causes the system to automatically apply the default gain when an implement is raised and/or when a steering oscillation has been detected and to automatically apply the user defined gain when the implement has been lowered and/or the GPS track has been acquired.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of applicant's co-pending applicationU.S. Ser. No. 10/919,874, filed 17 Aug. 2004 and titled VARIABLE GAINLOGIC FOR A GPS BASED AUTOMATIC STEERING SYSTEM, which application ispending.

FIELD OF THE INVENTION

The present invention relates generally to GPS based automatic vehiclesteering systems such as are used with agricultural tractors or otherwork vehicles. More particularly, the present invention relates to suchsteering systems which have a user adjustable gain for the closed loopsteering control system. Specifically, the present invention relates tologic for automatically applying a default gain or a user gain dependingupon various factors including sensed oscillations, implement position,soil condition etc.

BACKGROUND OF THE INVENTION

Due to the increasing popularity of GPS based automatic steering systemsin agricultural tractors, it has become important to develop a strategyto tune the closed loop steering control system for maximum accuracyover a wide range of applications. Some manufacturers today provide oneor more user adjustable gains that are used to tune the closed loopcontrol system for optimal performance when the implement is engaged inthe ground. However, the control gains obtained with the implement inthe ground are often too high when the automatic steering system isengaged while the implement is raised. This condition is common whenacquiring the GPS track or when crossing waterways or lanes in thefield. The high gains selected with the implement lowered in the groundfor maximum accuracy cause undesirable steering oscillations when theimplement is raised, especially on tractors with pneumatic tires. Thetractor dynamics change considerably when the implement is raised andthe high closed loop gains in the steering controller excite the tractoryaw resonant frequency. The steering control system gains may also betoo high if a field soil condition is encountered that is significantlydifferent than the condition present during the original tuning. Whensoil conditions change from soft to hard, the system damping decreasesallowing the closed loop steering controller to excite the tractor yawresonant frequency. If steering oscillations are present when acquiringthe GPS track with the implement raised, a longer distance will berequired for track acquisition. Planting and bedding applicationsrequire that the tractor be on track in the shortest distance possiblewhen turning around at the end of the field with the implement raised.When oscillations occur due to varying soil conditions, steeringaccuracy is reduced. In each of these cases, the operator is forced tomake a gain compromise between steering oscillations when the implementis raised and high accuracy when the implement is lowered. In somecases, the steering oscillation amplitude increases until the user isforced to turn the steering system off and lower the gain. Accordingly,a method is needed to automatically adjust the steering control systemgains during track acquisition, when the implement is raised, or whensteering oscillations are present. Further, this method must allow theuser to tune the steering system for maximum accuracy without thepenalty of poor system performance with a raised implement.

SUMMARY OF THE INVENTION

This invention consists of software logic implemented in a GPS basedautomatic steering system that automatically adjusts the steeringcontrol system gain during track acquisition and when steering systemoscillations are present due to a raised implement or changing soilconditions. This software runs in the steering controller, which is partof the steering system. Current systems do not employ logic to detectand/or limit steering oscillations.

It is well known that the gain must be adjusted to adapt an automaticsteering system to different implements, soil conditions, and vehicletire and ballast configurations. This user adjustable gain is alsorequired to maximize system accuracy with the implement engaged. Foroptimal performance, two levels of closed loop system gain are required.A default gain level is used when the implement is raised, and a useradjusted gain level is applied when the tractor is at work with theimplement lowered. This invention provides logic on when toautomatically apply the user gain or the default gain in the closed loopsteering controller.

Objects of the invention together with the advantages thereof over theknown art which will become apparent from the detailed specificationwhich follows are attained by a method for controlling steeringoscillations in a closed loop GPS based automatic steering system for avehicle, the closed loop steering system having a default gain and auser defined gain, comprising the steps of: monitoring both heading andlateral errors from a desired GPS track; automatically applying thedefault gain when the lateral and heading errors exceed a predeterminedthreshold indicating a steering oscillation; and, automatically applyingthe user defined gain when the GPS track has been acquired.

Other objects of the invention are attained by a method for controllingsteering oscillations in a closed loop GPS based automatic steeringsystem for a vehicle, the closed loop steering system having a defaultgain and a user defined gain, comprising the steps of: determiningwhether a lateral error and a heading error are below threshold valueswhich have been predetermined for the degree of accuracy desired;applying the default gain until the heading error is below thresholdvalues which have been predetermined for the degree of accuracy desired;and, applying a user gain level after the heading error is belowthreshold values which have been predetermined for the degree ofaccuracy desired and maintaining the user defined gain until steeringoscillations are detected or the closed loop steering control system isdeactivated.

Still other objects of the invention are attained by a method forcontrolling steering oscillations in a closed loop GPS based automaticsteering system for a vehicle, the closed loop steering system having adefault gain and a user defined gain, comprising the steps of:determining whether a positive heading threshold crossover has occurred;setting a crossover flag and negative timer if a positive headingthreshold crossover has occurred; determining whether both a positivecrossover has been seen and a positive timer is less than apredetermined period; incrementing the positive timer if both a positivecrossover has been seen and the positive timer is less than apredetermined period; determining whether the positive crossover timeris greater than a predetermined period; clearing the crossover flags andresetting the positive timer if the positive crossover timer is greaterthan a predetermined period; determining whether a negative headingthreshold crossover has occurred; setting the crossover flag andpositive timer if a negative heading threshold crossover has occurred;determining whether both a negative crossover has been seen and thenegative timer is less than a predetermined period; incrementing anegative crossover timer if both a negative crossover has been seen andthe negative timer is less than a predetermined period; determiningwhether the negative crossover timer is greater than a predeterminedperiod; clearing the crossover flags and resetting the negative timer ifthe negative crossover timer is greater than a predetermined period;determining whether both the lateral and heading errors are within apredetermined threshold; setting a line acquired flag if both thelateral and heading errors are within a predetermined threshold;determining whether the automatic steering system is disabled; clearingthe line acquired flag if the automatic steering system is disabled;applying a default gain if a steering oscillation is present; and,applying a user defined gain if no steering oscillation is present andthe track has been acquired.

Further objects of the invention are attained by a method of controllingthe gain of a closed loop GPS based automatic steering system having adefault gain mode and a user defined gain mode comprising the step of:applying the user defined gain to the closed loop steering controllerwhen an implement is lowered and the vehicle is at work.

Still further objects of the invention are attained by a method ofcontrolling the gain of a closed loop GPS based automatic steeringsystem having a default gain mode and a user defined gain modecomprising the step of: applying the default gain when an implement hasbeen raised.

Additional objects of the invention are attained by a method ofcontrolling steering oscillations of a vehicle using a closed loop GPSbased automatic steering system comprising the step of: monitoring alateral error and the amplitude and frequency of a heading error andapplying a default gain when the amplitude of the heading error exceedsa defined threshold, the oscillation period of the heading error signalis less than a defined period, and the lateral error is within a lateralerror window.

Other objects of the invention are attained by a method for controllingsteering oscillations in an agricultural vehicle having a GPS basedautomatic steering system, the steering system having a steeringcontroller controlling a valve for actuating a hydraulic cylinder usedto turn the vehicle; a flowmeter or wheel angle sensor provided betweenthe valve and the hydraulic cylinder to provide wheel angle informationto the steering controller; the steering controller communicating with adisplay and mobile processor unit serving as a user interface andproviding lateral and heading error information to the display; a GPSreceiver providing position information to the controller; a switchoperable to activate the steering system; and a steering wheel sensoroperable to automatically deactivate the system in response to operatormovement of the steering wheel, wherein a GPS track defines the desiredcourse for the vehicle, a lateral error is defined as the distance ofthe vehicle from the GPS track and a heading error is defined as theangle of the vehicle's actual track from the GPS track comprising thesteps of: determining whether the lateral error and heading error arebelow threshold values which have been predetermined for the degree ofaccuracy desired; applying the default gain until the heading error isbelow threshold values which have been predetermined for the degree ofaccuracy desired; applying a user gain level after these conditions aresatisfied and maintaining the user gain level until steeringoscillations are detected or the closed loop steering control system isdeactivated.

In general, a method is provided for controlling steering oscillationsin a work vehicle having a closed loop GPS based automatic steeringsystem, wherein the closed loop steering system has a default gain and auser defined gain. The method causes the system to automatically applythe default gain when an implement is raised and/or when a steeringoscillation has been detected and to automatically apply the userdefined gain when the implement has been lowered and/or the GPS trackhas been acquired.

To acquaint persons skilled in the art most closely related to thepresent invention, one preferred embodiment of the invention thatillustrates the best mode now contemplated for putting the inventioninto practice is described herein by and with reference to, the annexeddrawings that form a part of the specification. The exemplary embodimentis described in detail without attempting to show all of the variousforms and modifications in which the invention might be embodied. Assuch, the embodiment shown and described herein is illustrative, and aswill become apparent to those skilled in the art, can be modified innumerous ways within the spirit and scope of the invention—the inventionbeing measured by the appended claims and not by the details of thespecification.

BRIEF DESCRIPTION OF THE DRAWINGS

For a complete understanding of the objects, techniques, and structureof the invention reference should be made to the following detaileddescription and accompanying drawings, wherein:

FIG. 1 is schematic view of a work vehicle having a closed loop GPSbased automatic steering system;

FIG. 2 is a schematic illustration of a vehicle's lateral and headingerrors from a GPS track;

FIG. 3 illustrates a steering oscillation of a vehicle relative to a GPStrack;

FIG. 4 is a schematic illustration wherein the gain levels are plottedagainst the vehicle track; and,

FIG. 5 is a flow chart illustrating the logic of the method according tothe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference now to FIG. 1, it can be seen that an agriculturaltractor having a GPS based automatic steering system is illustratedschematically, and is designated generally by the numeral 10. A steeringcontroller 12 controls a valve 14 for actuating a hydraulic cylinder 16used to turn the vehicle. A flowmeter 18 may be provided between thevalve 14 and the hydraulic cylinder 16 to provide wheel angleinformation to the steering controller 12. Alternatively a wheel anglesensor 20 may be provided for this purpose. The steering controller 12communicates with a display and mobile processor unit 22 which serves asa user interface and provides lateral and heading error information tothe display 22. A GPS receiver 24 provides position information to thecontroller 12. A switch 26 activates the steering system, while asteering wheel sensor 28 automatically deactivates the system inresponse to operator movement of the steering wheel.

FIG. 2 illustrates the lateral and heading error variables used by theinvention to control an automatic steering system. A GPS track 30defines the desired course for the vehicle 10. The lateral error 32 isthe distance of the vehicle 10 from the GPS track 30. The heading error34 is the angle of the vehicle's actual track 33 from the GPS track 30.The GPS track 30 is considered to be acquired when the lateral error 32and heading error 34 are below threshold values which have beenpredetermined for the degree of accuracy desired. While these thresholdvalues can vary depending upon the degree of accuracy desired withoutdeparting from the spirit and scope of the invention, for purposes ofillustration the following description utilizes a lateral error 32 ofless than 0.25 meters and a heading error 34 of less than 0.75 degreesfor track acquisition. Those having skill in the art will recognize thatthe error values used for a particular application may be greater thanor less than those described here. During track acquisition, theinvention applies the default gain until the heading error is within0.75 degrees and the lateral error is within 0.25 meters. The user gainlevel is applied after these conditions are satisfied and remains ineffect until steering oscillations are detected or the closed loopsteering control system is deactivated at the end of the field.

Steering oscillations are detected by monitoring the lateral and headingerrors. FIG. 3 illustrates a steering oscillation condition 35 caused bythe closed loop steering controller with gains that are too high. Theheading and lateral errors for this oscillation are sinusoidal andcentered about zero. The frequency of this oscillation is a nominal 0.5Hz. It follows then that this type of steering oscillation can beidentified by monitoring the amplitude and frequency of the lateralerror, heading error, or wheel angle. The logic used for this inventionmonitors the amplitude and frequency of the heading error while thetractor is within a lateral error window. A steering oscillationcondition is defined when a heading error cycle meets the followingconditions: 1.) the amplitude of the heading error must exceed a definedthreshold; 2.) the oscillation period of the heading error signal mustbe less than a defined period; 3.) the lateral error is within a lateralerror window.

FIG. 4 is an illustration of a vehicle track plotted against the gainlevels applied by the method of the present invention. More particularlythe gain levels are illustrated generally at 50, with 52 representing alevel of zero gain, 54 representing the default gain level, and 56representing a user defined gain level. The gain levels 50 are plottedagainst a vehicle track 60 starting at 62. As shown, from the time thetrack is started at 62 until the GPS track 30 is acquired at 64 thedefault gain level 54 is applied by the invention. Once the GPS track 30is acquired at 64 the user defined gain level 56 is automaticallyapplied by the invention. As can be seen, at 66 a condition changecauses the vehicle 10 to begin to diverge from the GPS track 30. At 68the invention logic notes that a crossover of the positive threshold 70has occurred. Likewise at 72 the logic notes that a crossover of thenegative threshold 74 has occurred. At 76 a second crossover of thepositive threshold 70 is noted. At this point the logic recognizes thata steering oscillation condition exists and at 78 applies the defaultgain level 54. When the GPS track 30 is again acquired at 80 the userdefined gain level 56 is again applied by the invention logic. Thus,when a steering oscillation condition is present, the closed loopsteering controller gain is automatically set to the default value.After the steering condition has settled for the defined period, theuser gain will be automatically applied to the closed loop steeringcontroller providing that the line acquired condition is satisfied.

The steps for determining whether a steering oscillation is present andapplying the appropriate gain are illustrated in the flowchart of FIG.5. First the static variables are initialized at 100. The pseudo codelisting for this step is:

negative_crossover_seen = FALSE positive_crossover_seen = FALSEpast_positive_crossover_seen = FALSE past_negative_crossover_seen =FALSE positive_timer = 0 negative_timer = 0 line_acquired = FALSE

Variables from other functions are acquired at 102 . These variablesinclude the function task rate in seconds, whether automatic steering isactive, the default gain, the user defined gain, the lateral error andthe heading error. The pseudo code listing for this step is:

function_taskrate_in_seconds automatic_steering_active default_gainuser_defined_gain lateral_error heading_error

At 104 a determination is made as to whether a positive headingthreshold crossover has occurred. If a positive heading thresholdcrossover has occurred the crossover flag and negative timer are set at106. The pseudo code listing for steps 104 and 106 is:

 If (heading_error > 1.5 degrees) & (abs(lateral_error) < 0.5 meters) &(positive_crossover_seen = FALSE)  then:   positive_crossover_seen =TRUE   past_negative_crossover_seen = negative_crossover_seen  negative_crossover_seen = FALSE   negative_counter = 0

Those having skill in the art will recognize that the heading andlateral error values used in the pseudo code listing are set accordingto the particular application and the values listed here are forpurposes of illustration only and can be replaced by any other valuepractical to the particular application without departing from thespirit and scope of the invention.

Next at 108 a determination is made whether both a positive crossoverhas been seen and the positive timer is less than a predeterminedperiod. If so, the positive crossover timer is incremented at 110. Thepseudo code for these steps is:

 If (positive_crossover_seen = TRUE) and (positive_timer < 2.5 seconds) then:   positive_timer = positive_timer + function_taskrate_in_seconds

As with the heading and lateral error thresholds the time threshold canbe any value which is practical to the application. The threshold of 2.5seconds included in the pseudo code is for the purpose of illustration.

A determination as to whether the positive crossover timer is greaterthan a predetermined period is next made at 112. If the positivecrossover timer is greater than the predetermined period, the crossoverflags are cleared and the positive timer is reset at 114. The pseudocode for steps 112 and 114 is:

 If positive_timer >= 2.5 seconds then:  positive_crossover_seen = FALSE past_negative_crossover_seen = FALSE  past_positive_crossover_seen =FALSE  positive_timer = 0

Next the logic determines whether a negative heading threshold crossoverhas occurred at 116. If a negative heading threshold crossover hasoccurred the crossover flag and positive timer are set at 118. Thepseudo code listing for these steps is:

 If (head_error < −1.5 degrees) and (abs(lateral_error) < 0.5 meters)and (negative_crossover_seen = FALSE)  then:   negative_crossover_seen =TRUE   past_positive_crossover_seen = positive_crossover_seen  positive_crossover_seen = FALSE   positive_timer = 0

A determination is next made at 120 whether both a negative crossoverhas been seen and the negative timer is less than a predeterminedperiod. If so, the negative crossover timer is incremented at 122. Thepseudo code for these steps is as follows:

 If (negative_crossover_seen = TRUE) & (negative_timer < 2.5 seconds) then:  negative_timer = negative_timer + function_taskrate_in_seconds

A determination as to whether the negative crossover timer is greaterthan a predetermined period is next made at 124. If the negativecrossover timer is greater than the predetermined period, the crossoverflags are cleared and the negative timer is reset at 126. The pseudocode for steps 124 and 126 is:

If negative_timer >= 2.5 seconds then:   negative_crossover_seen = FALSE  past_positive_crossover_seen = FALSE   past_negative_crossover_seen =FALSE   negative_counter = 0

At 128 a determination is made whether both the lateral and headingerrors are within a predetermined threshold. If so the line acquiredflag is set at 130. The pseudo code for these steps is:

 If (line_acquired = FALSE) & (abs(lateral_error) < 0.25 meter) &(abs(heading_error) < 0.75 degrees)  then:   line_acquired = TRUE

If at 132 the auto steering system is disabled, the line acquired flagis cleared at 134. The pseudo code for steps 132 and 134 is as follows:

If automatic_steering_active = FALSE then:   line_acquired = FALSE

If at 136 an oscillation is present and/or the GPS track has not beenacquired the logic applies the default gain at 138. If, however, nooscillation is present and the track has been acquired, the logicapplies the user defined gain at 140. The pseudo code for these stepsis:

 If(past_negative_crossover_seen = TRUE) & (past_positive_crossover_seen= TRUE) or (line_acquired = FALSE)  then:   gain = default_gain  line_acquired = FALSE  else:   gain = user_defined_gain

When the above logic sequence has been completed at 142 the sequencebegins again at 102.

In addition to the above logic, the user gain level can be applied tothe closed loop steering controller, for example, when any of thefollowing conditions indicate that an implement is lowered and thevehicle is at work: (This information is available to the steeringcontroller over the vehicle's CAN bus) 1. ) the hitch position sensorreading drops below a user defined threshold; 2. ) the hitch draftsensor reading exceeds a user defined threshold; 3. ) the implementposition sensor reading on the electro-hydraulic depth control (EHDC)equipped implements reaches the lowered position set point; 4. ) theuser designated SCV (selective control valve) lower switch is activated;5. ) the hitch lower switch is activated; 6. ) the engine load signalincreases above a user defined threshold.

Similarly, the default gain level can be applied to the closed loopsteering controller when any of the following conditions indicate thatthe implement has been raised: (This information is available to thesteering controller over the vehicle's CAN bus) 1.) the hitch positionsensor reading rises above a user defined threshold; 2.) the hitch draftsensor reading drops below a user defined threshold; 3.) the implementposition sensor reading on the EHDC equipped implements reaches theraised position set point; 4.) the user designated SCV raise switch isactivated; 5.) the hitch raise switch is activated; 6.) the engine loadsignal drops below a user defined threshold.

This method of changing the steering control system gains allows theuser to optimize steering accuracy without affecting line acquisitionperformance. Also, when soil conditions change or the implement israised, steering oscillations are limited resulting in improvedaccuracy. Finally, the operator now has time to make any required gainadjustments without having to turn the steering system off due tounbounded steering oscillations.

Thus it can be seen that the objects of the invention have beensatisfied by the structure presented above. While in accordance with thepatent statutes, only the best mode and preferred embodiment of theinvention has been presented and described in detail, it is not intendedto be exhaustive or to limit the invention to the precise formdisclosed. Obvious modifications or variations are possible in light ofthe above teachings. The embodiment was chosen and described to providethe best illustration of the principles of the invention and itspractical application to thereby enable one of ordinary skill in the artto utilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. All suchmodifications and variations are within the scope of the invention asdetermined by the appended claims when interpreted in accordance withthe breadth to which they are fairly and legally entitled.

1. A method of controlling the gain of a closed loop GPS based automaticsteering system having a default gain mode and a user defined gain modecomprising the step of: applying the user defined gain to the closedloop steering controller when an implement is lowered and the vehicle isat work.
 2. A method of controlling the gain of a closed loop GPS basedautomatic steering system as described in claim 1 wherein the userdefined gain is applied when a hitch position sensor reading drops belowa user defined threshold indicating that the implement has been lowered.3. A method of controlling the gain of a closed loop GPS based automaticsteering system as described in claim 1 wherein the user defined gain isapplied when a hitch draft sensor reading exceeds a user definedthreshold indicating that the implement has been lowered.
 4. A method ofcontrolling the gain of a closed loop GPS based automatic steeringsystem as described in claim 1 wherein the user defined gain is appliedwhen an implement position sensor reading on an electro-hydraulic depthcontrol (EHDC) equipped implement reaches a lowered position set pointindicating that the implement has been lowered.
 5. A method ofcontrolling the gain of a closed loop GPS based automatic steeringsystem as described in claim 1 wherein the user defined gain is appliedwhen a user designated selective control valve lower switch is activatedindicating that the implement has been lowered.
 6. A method ofcontrolling the gain of a closed loop GPS based automatic steeringsystem as described in claim 1 wherein the user defined gain is appliedwhen a hitch lower switch is activated indicating that the implement hasbeen lowered.
 7. A method of controlling the gain of a closed loop GPSbased automatic steering system as described in claim 1 wherein the userdefined gain is applied when an engine load signal increases above auser defined threshold indicating that the implement has been lowered.8. A method of controlling the gain of a closed loop GPS based automaticsteering system having a default gain mode and a user defined gain modecomprising the step of: applying the default gain when an implement hasbeen raised.
 9. A method of controlling the gain of a closed loop GPSbased automatic steering system as described in claim 8 wherein thedefault gain is applied when a hitch position sensor reading rises abovea user defined threshold indicating that the implement has been raised.10. A method of controlling the gain of a closed loop GPS basedautomatic steering system as described in claim 8 wherein the defaultgain is applied when a hitch draft sensor reading drops below a userdefined threshold indicating that the implement has been raised.
 11. Amethod of controlling the gain of a closed loop GPS based automaticsteering system as described in claim 8 wherein the default gain isapplied when an implement position sensor reading on the EHDC equippedimplement reaches a raised position set point indicating that theimplement has been raised.
 12. A method of controlling the gain of aclosed loop GPS based automatic steering system as described in claim 8wherein the default gain is applied when a user designated selectivecontrol valve raise switch is activated indicating that the implementhas been raised.
 13. A method of controlling the gain of a closed loopGPS based automatic steering system as described in claim 8 wherein thedefault gain is applied when a hitch raise switch is activatedindicating that the implement has been raised.
 14. A method ofcontrolling the gain of a closed loop GPS based automatic steeringsystem as described in claim 8 wherein the default gain is applied whenan engine load signal drops below a user defined threshold indicatingthat the implement has been raised.